In electronic circuits it is frequently desirable to modify the frequency of a periodic signal such as a clock signal or local oscillator output. For example, in devices such as wireless transceivers, a voltage-controlled oscillator (VCO) is often used to provide a periodic signal for downconverting received RF signals, upconverting signals to RF for transmission, as the basis for clock signals, and the like. Generally the VCO output cannot be used directly as a clock signal due to power amplifier pulling. That is, some of the power may leak from the power amplifier to the VCO. If the VCO and power amplifier run at similar frequencies, the VCO frequency may be pulled away from its center frequency and towards the power amplifier frequency. In addition, multiple clocks at different frequencies may be required to support multiple standards in a single device.
Frequency multiplication is often used to accomplish such modifications. One common application is frequency tripling, where a circuit triples the frequency of an input signal. FIG. 1 shows a conventional circuit that is widely used for frequency tripling. The circuit of FIG. 1 includes a capacitor C1, a resistor R1, a bipolar junction transistor Q1, and a tank circuit comprising an inductor L1 and a variable capacitor C2. A sinusoidal input signal Sin having an input frequency Fin is applied to the circuit. Transistor Q1 is chosen such that the input signal Sin drives transistor Q1 into a non-linear region of operation where higher-order harmonics are generated. The value of variable capacitor C2 is tuned so that the tank circuit acts as a bandpass filter to pass the third harmonic Fout=3Fin in output signal Sout.
One disadvantage of the approach of FIG. 1 is that substantial power is required to drive transistor Q1 strongly into the non-linear operating region. Therefore this approach is not suitable for battery-powered devices such as mobile telephones and the like. Another disadvantage of the approach of FIG. 1 is that the spectrum of the output signal is rich in unwanted frequency components referred to as “spurs.” In particular, output signal Sout includes a strong spur at the input frequency.
In some cases it is desirable to increase the frequency of a signal fractionally. FIG. 2 shows a conventional circuit for multiplying the frequency of a signal by 3/2. In the circuit of FIG. 2, a periodic input signal Sin having a frequency Fin that is ⅔ of the desired output frequency Fout is applied to a divider 202. Divider 202 divides the frequency Fin of signal Sin by 2 such that the signal Sdiv output by divider 202 has a frequency Fin/2=Fout/3. Multiplier 204 mixes the input signal Sin with the signal Sdiv output by divider 202. This mixing generates two tones (2Fout/3+/−Fout/3), one tone at Fout/3 and the other at Fout. An LC tank at the output of multiplier 204 acts as a bandpass filter to pass the Fout tone and to suppress the Fout/3 tone. Therefore the signal Sout output by multiplier 204 has a frequency Fout=3Fin/2.
One disadvantage of the approach of FIG. 2 is that the circuit produces a spur at the frequency Fout/3. Because the output of divider 202, and one input of multiplier 204, operate at the frequency Fout/3, a spur at frequency Fout/3 is generated in signal Sout by mixing and coupling. Such spurs also appear in the supply/substrate current, and so are propagated to the rest of the circuit or chip, where the spurs can reduce performance cause circuit malfunctions, and the like.